Supercritical water oxidation (SCWO) has been demonstrated to be effective in the destruction of hazardous and toxic wastes. This chemical oxidation process takes place at temperatures above 374.15.degree. C. and 22.1 MPa. High destruction efficiencies (&gt;99.99%) over relatively short residence times (on the order of seconds to minutes) are achieved due to the high solubility of organic compounds and oxygen, alleviating mass transfer hindrances. Residual organic compounds, such as acetic acid, are relatively harmless and may be treated readily by conventional treatment techniques.
SCWO is currently being investigated to address the disposal of chemical warfare agents and munitions which are stored at several sites within and outside the U.S. The stockpile of chemical agents and munitions consists of several nerve and blistering agents as well as explosives or energetic materials. Dimethyl methylphosphonate, DMMP (C.sub.4 H.sub.10 FO.sub.2 P), is a simulant selected by the Department of the Army to study the behavior of the nerve agent GB, or Sarin (C.sub.4 H.sub.10 FO.sub.2 P). These two compounds are similar in structure and properties; DMMP may be used to study the potential fate of GB during SCWO, without producing harmful by-products, in particular, hydrogen fluoride gas. The present invention resulted from the study of DMMP under SCWO conditions.
FIG. 1 summarizes the reaction pathways for hydrolysis and oxidation of DMMP and its by-products. DMMP is base-hydrolyzed to form MPA and methanol at ambient conditions (Hudson et al., 1956). This pathway was confirmed in a kinetic study of hydrolysis/pyrolysis runs of DMMP in SCWO (Turner, 1993). As shown in equation A of FIG. 1, methanol and MPA were observed to form in a 2:1 ratio.
MPA may undergo hydrolysis or oxidation in supercritical water. The structure of MPA appears to meet criteria set in previous studies for compounds which are readily hydrolyzed in supercritical water (Klein et al., 1990; Townsend et al., 1988). Since the C atom is saturated and bonded to the heteroatom phosphorus, attack appears to occur at the P--C bond, producing phosphoric acid and methane as shown in equation B of FIG. 1. At least one previous study did not show this reaction pathway to be significant in kinetic studies of the supercritical water oxidation of DMMP (Turner, 1993). Significant quantities of methane do not appear to have been detected in hydrolysis and oxidation tests performed at supercritical temperatures.
Free radical oxidation of MPA was studied at ambient conditions by Mill and Gould (1979). Oxidation of MPA appears to have been found to produce carbon dioxide, water, phosphates and trace amounts of CO. Kinetic studies conducted by Turner (1993) also stated carbon dioxide, water, phosphates, carbon monoxide as well as methanol as by-products of MPA oxidation in supercritical water as shown in equation C of FIG. 1.
Oxidation of methanol may result in the intermediate production of carbon monoxide and water and further reactions producing carbon dioxide and hydrogen may occur (Webley and Tester, 1989). The concentration (% by volume) of off-gas appears to have been less than 0.01. However, significant hydrolysis of methanol does not appear to have been observed in kinetic studies by Turner (1993). The oxidation of methanol is shown in equations E, F and G in FIG. 1. Carbon monoxide oxidation in supercritical water is shown in equation G (Holgate et al., 1992). A parallel water-gas shift reaction, not shown in FIG. 1, was observed to occur. The water-gas shift reaction occurs in the absence of oxygen (typical in reactor heat-up conditions) whereby CO reacts with water to form CO.sub.2 and H.sub.2. Researchers found that 1.4-23% of CO conversion was due to the water-gas shift reaction. Methane oxidation in supercritical water has been studied by Webley and Tester (1991). As indicated in equation D of FIG. 1, carbon dioxide was a major by-product found, although trace amounts of carbon monoxide and hydrogen were detected. Hydrogen generation was attributable to water-gas shift reactions.
The properties of supercritical water have been demonstrated to change in the presence of additives. By altering the reaction medium, reaction mechanisms may be modified. For example, typical reactions which take place in supercritical water are homolyric in nature, whereby free radicals are formed. This is due to several properties of supercritical water, in particular, its low dielectric constant, dissociation constant and low density. Homolyric reaction mechanisms are not very selective, accounting for the high reactivity of free radicals. Heterolytic reaction mechanisms, on the other hand, are ionic in nature and are readily acid- or base-catalyzed. Hydrolysis reactions are typically heterolytic mechanisms. Because of their ionic nature and the non-polar environment of supercritical water, heterolytic reactions are not readily supported in supercritical water. However, by altering the properties of supercritical water, hydrolysis reactions may be enhanced. Heterolytic chemistry has been shown to prevail when K.sub.w .gtoreq.10.sup.-14 while homolyric chemistry (free radical) prevails when K.sub.w &lt;&lt;10.sup.-14 (Antal et al., 1987).
At densities above 0.3 g/mL to 0.4 g/mL and at temperatures lower than 500.degree. C., supercritical fluid retains its ionic properties (Antal et al., 1987; Xu et al., 1990). Hydrolysis and oxidation of acetamide in supercritical water was studied by Lee and Gloyna, (1990) at densities between 0.090 and 0.135 g/mL. The dielectric constant was varied between 1.5 and 5.5 and K.sub.w was varied between 10.sup.-16.5 and 10.sup.-24.5 Researchers indicated no effect of .epsilon. or K.sub.w on the hydrolysis of acetamide. Therefore, it was concluded, there is little dependence of hydroxyl or hydronium ions on the reaction since the concentration of these ions was so small. Since it was believed that ionic mechanisms responsible for the observations could not be supported in the low density environment due to the lack of an ionized species required for nucleophilic attack, it was proposed that the water molecule serves as the nucleophile in acetamide hydrolysis. Others proposed this mechanism for the hydrolysis of dibenzyl ether (DBE) and phenethyl phenyl ether (PPE) (Klein et al., 1990).
Pyrolysis and hydrolysis of dibenzyl ether (DBE) and benzyl phenyl amine (BPA) in supercritical water appeared to indicate an increase in the selectivity of the hydrolysis by-products upon the addition of sodium chloride (Torry et al., 1991). The selectivity was observed up to a certain concentration of salt when the presence of a second phase inhibited hydrolysis. Further, increased water density was found to increase the dielectric constant in supercritical water and stabilize the polar hydrolysis transition state over less-polar reactants (Townsend and Klein, 1985). Hydrolysis studies of guaiacol in supercritical water showed the ability of supercritical water to support heterolytic chemistry in addition to free radical mechanisms (Huppert et al., 1989).
Japanese Patent JP 5031000 relates to a method comprising selectively hydrolysing and/or pyrolysing natural or synthetic high molecular compounds using water under supercritical or subcritical conditions as solvent. British Patent BR 8204075 relates to the production of synthesis gas carried out by reacting a hydrocarbon with steam and optional O.sub.2 in a steam reformer or partial oxidation gasifier. The reported improvement relates to reducing and controlling the H.sub.2 /CO ratio in the synthesis gas.
German Patent DE 4215087 relates to the recovery of caprolactam monomer from polycaprolactam by high pressure hydrolysis of the polymer with 5-30 wt. % water at 200.degree.-350.degree. C. followed by recovery of monomer from the resulting aqueous solution or suspension, including hydrolysis reportedly effected in the presence of an alkali hydroxide at pH 5-10. WO 9322490 relates to the recovery of inorganic processing chemicals, in the form of bicarbonates and/or carbonates from waste liquid in the production of cellulose by the organo-solvent process involving partial or complete oxidation of organic components in the aqueous phase with air and/or oxygen.
U.S. Pat. No. 5,133,877 relates to conversion of hazardous materials using supercritical water oxidation. U.S. Pat. No. 4,483,761 relates to the addition of light olefins in the cracking of hydrocarbons. U.S. Pat. No. 3,984,311 relates to the use of a mixture of nitrate and iodide or bromide ions in wet oxidation. U.S. Pat. No. 4,212,735 relates to the addition of a transition metal ion to the nitrate/iodide (bromide) system. U.S. Pat. No. 5,232,604 refers to the nitrate/iodide (bromide) system as undergoing oxidation/reduction reactions and are therefore reactants. This patent also refers to the reported addition of a caustic material such as NaOH to a reactor for neutralizing acids and producing salts which precipitate in the reactor. Methane was reportedly produced in catalyzed hydrothermal treatment of p-cresol and methyl isobutyl ketone (Baker et al., 1989).
Wet oxidation processes (both supercritical and subcritical water oxidation) convert complex organic compounds into simpler structured compounds. Typically, at supercritical condition, the complete conversion results in carbon dioxide, water, and mineral acids. An ideal wet oxidation process operating at near self-sustaining conditions typically requires an organic load of 5 wt % (about 50 g/L chemical oxygen demand). However, wet oxidation of relatively concentrated organic wastes imposes three basic concerns: (1) excessive corrosion by the mineral acids produced from the process; (2) long reactor residence times to achieve high organic conversions; and (3) increased consumption of high-pressure oxygen.
A goal of the present invention is control of the wet oxidation process so that incompletely oxidized intermediate compounds are optimally produced and recovered.